There is a need in the field of industrial hygiene for a colorimetric, passive device, which may be carried by an individual in the form of a badge, to provide a direct-read quantitative measurement of the range of concentration of various toxic gases and/or vapors in the ambient atmosphere, by means of real-time integration of the exposure level of such toxic gases or vapors over a fixed time horizon.
Several methods are presently known for the colorimetric estimation of time-weighted average (TWA) measurements and the short-term exposure limit (STEL) of toxic gases and vapors in the atmosphere. These methods can be classified into the following major categories:
1. Methods which compare the quality of a received colorimetric result (density, color, tone, shade, or nuance) to a given color standard. The colorimetric devices of this category rely on a gas pollutant reacting chemically with reagent(s) dispersed on a suitable substrate to produce a color change. The estimation of the gas pollutant is semiquantitative and is reliable only for a relatively narrow exposure interval. PA1 2. Methods which use the quantity of the received color expressed by area or length of stain. In general, the devices in this category are made of transparent tubes filled with a granular, solid support impregnated with colorimetric reagent(s). Gas pollutants penetrate by diffusion through a diffusion retainer and react with the colorimetric reagent(s). The colorimetric result is a length of stain proportioned to the total exposure value of the concentration multiplied by the exposure time, i.e., CT value. The measure of exposure is visual and occurs by comparing the length of the stain to a fixed scale. Although this is a significant advantage, such devices are nonlinear and their response time changes with the length of stain.
Several passive, direct-read, colorimetric devices have been proposed in the patent literature, which rely upon the degree of gas penetration through the chromophoric reagent to determine dosage exposure, and to visually, provide a graded sensitivity in the color detection for real-time measurement. In the dosimeter of U.S. Pat. No. 4,478,792, issued to McConnaughy, a stack of porous sheets are impregnated with a reagent, causing the sheets in the stack to sequentially change color upon exposure to a gas pollutant relative to the degree of gas penetration. The degree of gas penetration is a function of the gas concentration in the atmosphere over a given time period. By removing the sheets at the end of the time period, the last color-changed sheet corresponds to the highest dosage exposure level. This is a cumbersome and impractical way to measure dosage exposure, as well as being unreliable. In U.S. Pat. No. 4,271,121, issued to Diller, et al., the indicator layer containing the colorimetric reagent is subdivided into a plurality of measuring fields. A graded sensitivity in color detection is achieved, using a plurality of membranes of filter paper superimposed over the indicator layer in a staggered relationship, or by using different membrane thicknesses. The variation in membrane thickness or the staggered membrane arrangement varies the diffusion resistance to the separate measuring fields in the indicator layer and, accordingly, the degree of gas penetration to each measuring field which, in turn, varies the level of dosage exposure in each of the measuring fields. In theory, the arrangement, as taught by Diller, will provide a rapid visual, quantitative recognition of dosage exposure by providing different exposure ranges correlated to the different measuring fields. However, the range of color sensitivity is much too limited for practical use because the thickness of the membranes or the staggered arrangement of membranes needed to distinguish one measuring field from another varies in a geometric progression. Since a multiple number of measuring fields would be necessary to cover a reasonable exposure range for any known polluting gas, the variation in thickness or number of membranes would make the device unwieldy and far too expensive to manufacture.